Device for measuring gas concentration having dual emitter

ABSTRACT

A device for measuring the concentration of a gas contained in a cavity and for checking the operation of a catalytic element in an exhaust line in an automobile vehicle. A first emitter (E 1 ) composed of an optical pumped micro-cavity and for which the emission spectrum is within the gas absorption band emits a first radiation that passes through the cavity. A second emitter emits a second radiation that passes through the cavity. A receptor measures the optical intensity (I) of the radiation that passed through the cavity.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International PatentApplication No. PCT/FR02/00025, entitled “Device for Measuring GasConcentration” by Pascal Besesty, Engin Molva and Emanuel Hadji, whichclaims priority to French application no. 01/00132, filed on Jan. 5,2001, and which was not published in English.

TECHNICAL FIELD AND PRIOR ART

This invention relates to a gas concentration measurement device.

The invention is applicable in different fields, for example such as theanalysis of gaseous industrial waste, analysis of exhaust gas fromautomobile vehicles, control of pure air inlet in closed containments,control of smells, etc.

One particularly advantageous application of the device according to theinvention in the automotive field is for checking correct operation ofthe catalytic element of the exhaust system of a vehicle.

Different optical gas detection devices are known in prior art. Forexample, there are devices using laser emission diodes, light emittingdiodes (LEDs), and lamps.

The document “Remote Sensing of Methane Gas by Differential AbsorptionMeasurement Using a Wavelength Tunable DFB LD” (Y. Shimose et al., IEEEphotonics technology letters, vol. 3, No. 1, p. 86, January 1991) andthe document “Remote Detection of Methane with a 1.66 μm Diode Laser”(K. Uehara et al., Applied Optics, vol. 31, No. 6, p. 809, February1992) disclose devices making use of laser diodes.

This type of device uses complex electronic processing circuits that areexpensive due to the use of diodes with a very low wavelength (typicallyof the order of 1.65 μm). Therefore, these devices are not suitable foruse in fields of application for the general public, for example such asthe automotive field.

Known devices that use light emitting diodes have the advantage thatthey work at longer wavelengths between 3 and 6 μm. However, due to thevery broad spectral width of the radiation emitted by the diodes,interference filters have to be used that result in low emissionefficiency in the usage area. Moreover, this type of interference filteris expensive. There are other disadvantages with the use of lightemitting diodes. Thus, drifts in the radiation emitted by diodes due totemperature variations are high, and temperature compensation circuitshave to be used. Similarly, the transmission of radiation emitted bydiodes varies considerably and optical corrections are necessary.

The following documents disclose optical detection devices that uselight emitting diodes:

-   -   “Efficient 3.3 μm light emitting diodes for detecting methane        gas at room temperature”, M. K. Parry et al., Electronics        letters, vol. 30, No. 23, p. 1968, November 1994,    -   “InAsSb light emitting diodes and their applications to infrared        gas sensors”, W. Dobbelaere et al., Electronics letters, vol.        29, No. 10, p. 890, May 1993,    -   “Efficient 4.2 μm light emitting diodes for detecting CO ₂ at        room temperature”, Y. Mao et al., Electronics letters, vol. 32,        No. 5, p. 479, February 1996,    -   “High power 4.6 μm LEDs for CO detection grown by LPE”, A. Krier        et al., Electronics letters, vol. 35, No. 19, p. 1665, Sept.        1999.

Lamps (hot filament) also have the advantage that they work in the 3 to6 μm range. However, due to the very broad spectral width of the emittedradiation, it is also necessary to use interference filters. Theemission efficiency is even lower than the emission efficiency of lightemitting diodes. Furthermore, the emission also varies considerably andan optical correction has to be made. Furthermore, a mechanical chopperis necessary if signal processing requires amplitude modulation.

In summary, none of the emitters mentioned above are suitable for makinga compact, inexpensive and easy to use detection device. Furthermore,none of these devices can be used for simultaneous detection of severaldifferent gases.

The invention does not have the disadvantages mentioned above.

PRESENTATION OF THE INVENTION

This invention relates to a device for measuring the gas concentration,comprising:

-   -   a cavity containing at least one gas for which the concentration        is to be measured,    -   at least one first emitter composed of an optical micro-cavity        pumped by optical pumping means and for which the emission        spectrum is within the gas absorption band,    -   at least one second emitter composed of an optical micro-cavity        pumped by optical pumping means and for which the emission        spectrum is outside the gas absorption band,    -   reception means for measuring the optical intensity of a first        radiation output from the first emitter and transmitted through        the cavity, and the optical intensity of a second radiation        output from the second emitter and transmitted through the        cavity, and    -   a processing circuit for measuring the gas concentration        starting from the optical intensity of the first radiation and        the optical intensity of the second radiation.

The cavity may be an open or closed cavity. An “open” cavity means acavity that includes openings enabling gas to be entrained in a flux. A“closed” cavity means a cavity that does not include such openings.

The invention also relates to a device for checking operation of acatalytic element in an exhaust line in an automobile vehicle,characterized in that it comprises a device for measuring the gasconcentration according to the invention.

BRIEF DESCRIPTION OF THE FIGURES

Other specific features and advantages of the invention will becomeclear after reading one preferred embodiment made with reference to theappended figures, wherein:

FIG. 1 shows a principle diagram for a device for measuring the gasconcentration according to the invention;

FIG. 2 shows a principle diagram of an improvement to the device formeasuring the gas concentration shown in FIG. 1;

FIG. 3 shows a first example embodiment of a gas concentrationmeasurement device according to the invention;

FIG. 4 shows a second example embodiment of a gas concentrationmeasurement device according to the invention;

FIG. 5 shows a third example embodiment of a gas concentrationmeasurement device according to the invention.

The same marks represent the same elements in all figures.

DETAILED PRESENTATION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows a principle diagram of a gas concentration measurementdevice according to the invention. The device comprises a cavity Ccontaining a gas for which the concentration is to be measured, a firstradiation emitter E1, a second radiation emitter E2, a first receptionmeans R1, a second reception means R2, and an electronic processingcircuit T.

The emission spectrum of the emitter E1 is within the absorption band ofthe gas to be detected whereas the emission spectrum of the emitter E2is outside the absorption band of the gas to be detected. Radiation 11and 12 emitted by emitters E1 and E2 respectively pass through thecavity C over a distance d to form detected radiation t1 and t2respectively detected by reception means R1 and R2 respectively beyondthe cavity. The reception means R1 outputs a measurement I of theoptical intensity of the radiation t1 and the reception means R2 outputsa measurement I₀ of the optical intensity of the radiation t2. Aprocessing circuit T outputs the measurement of the concentration N ofthe gas starting from measurements of the optical intensity I and I₀.

The result is:

$\frac{I}{I_{0}} = {\exp\left( {{- \alpha} \times d} \right)}$where α=a×N, where a is the density of the gas in m⁻¹ ppm⁻¹, N is thegas concentration in ppm, where d is the length of the path of theoptical beam in the gaseous medium as mentioned above.

We can then write:

${\frac{I}{I_{0}} \approx {1 - {\alpha \times d}}},$or

${{\alpha \times d} \approx {1 - \frac{I}{I_{0}}}},$and therefore

${{\alpha \times d} \approx \frac{\delta\; I}{I_{0}}},$where δI=I₀−I.

The conclusion is:

$N = {\frac{1}{a \times d} \times {\frac{\delta\; I}{I_{0}}.}}$

Each of the emitters E1 and E2 comprises a resonant optical micro-cavityin which the active region is a heterostructure with semiconductor thatemits light at a wavelength determined by the choice of thesemiconductor and the type of heterostructure. The active layer is madeusing the epitaxy technique with semiconducting materials such as forexample CdHgTe, GaAlN, AlBN, GaAlAs, GaAsSb, GaAlSb, etc. or withdifferent families of semiconducting alloys in the II–VI family(compounds of Cd, Zn, Hg, Mn, Mg with Se, S, Te), or the III–V family(Ga, Al, In, B with N, As, P, Sb).

In general, heterostructures are formed by stacking multi-layers ofalloys on a substrate. The active zone may comprise quantum wells thatthen form light emitting zones. Epitaxy is done using known “molecularbeam epitaxy”, “organometallic epitaxy”, or “liquid phase epitaxy” typemeans.

In the micro-cavity emitter, the active zone made with semiconductingmaterials described above is located inside an optical micro-cavitycomposed of a Fabry-Perrot type cavity containing two mirrors. TheFabry-Perrot cavity is calculated so that optical resonance of thecavity corresponds to the emission wavelength of the semiconductor. Theresonant optical micro-cavities (of the Fabry-Perrot type) are alsoknown to experts in the subject.

The use of a resonant optical micro-cavity considerably improves theperformances of the emitter compared with an emission without a resonantmicro-cavity. The advantages related to use of a resonant micro-cavityare as follows:

-   -   improvement in the spontaneous emission and the emitted light        quantity (increase by a factor equal to approximately 10),    -   spectral refinement of the emission (the emission spectrum is        refined by a factor of 10 to 20),    -   better directivity (reduction in the divergence by an angle of        about 20°),    -   very large reduction in the dependence of the emission        wavelength on temperature (reduction by a factor of 100).

Optical pumping necessitates a source with a wavelength less than thewavelength of the emitter so that it can be absorbed by the active zoneof the semiconductor. For example, for infrared emitters based on CdHgTeemitting in the 3–5 μm range, a laser diode or a light emitting diodemay be used, for example emitting at 780 nm, 800 nm or 980 nm.Advantageously, there is no need to regulate the emission wavelength ofthe optical pump. This very much simplifies the device, since there isno need for a temperature regulation.

The emission power is proportional to the power of the pump. Forexample, it may be between 1 and 100 microWatts at ambient temperature.For example, for the application considered here, we will use a laserdiode to optically excite the emitters.

The input mirror to the optical micro-cavity is designed to betransparent to excitation wavelengths of the beam of the pump laserdiode. This is made conventionally with a dichroic mirror, with atransparency band at excitation wavelengths and a high reflectivity atthe wavelength of the emitter.

It is also possible to increase the sensitivity of the device by usingdedicated electronics. The light intensity of emission beams frominfrared emitters E1 and E2 can be varied by modulating the beam outputfrom the optical pumping element. If it is considered that the pumpingelement is a laser diode, the optical output beam from the infraredemitter can be modulated with a frequency of more than 100 MHz.Electronic filter functions (possibly the synchronous detectionfunction) can be controlled through this property, to select the usefulsignal to be detected (improvement of the signal to noise ratio). Ingeneral, each reception means comprises an interference filter to selectlight to be received. In the case of synchronous detection, it is thenpossible to eliminate this filter from the reception means. For example,by using encoded modulation, a single reception element can beselectively activated according to the activated emitter. Thisembodiment of the invention is shown in FIG. 2, in which an encodedmodulation command Mod is applied to a single reception means R.

According to the invention, the fact of making a differentialmeasurement between a useful signal measurement and a reference signaladvantageously reduces ambient parasite noise and eliminates temperaturedrifts from measurement systems.

FIG. 3 shows a first example embodiment of the gas concentrationmeasurement device according to the invention.

The device comprises four emitters E1, E2, E3, E4 and four detectingdiodes D1, D2, D3, D4. Radiation li (i=1, 2, 3, 4) output from emitterEi is coupled to the cavity C by a lens Lli. Radiation ti (i=1, 2, 3, 4)output from cavity C is coupled to detector Di by a lens Lti. Theelectrical signals output from detectors Di (i=1, 2, 3, 4) aretransmitted to the processing circuit T.

The emission spectrum from emitter E1 is located in the absorption bandof a first gas to be detected and the emission spectrum of emitter E3 islocated in the absorption band of the second gas to be detected. Theemitter E2 is associated with emitter E1 to measure the concentration ofthe first gas and the emitter E4 is associated with the emitter E3 tomeasure the concentration of the second gas.

The device as shown in FIG. 3 comprises four emitters, and can be usedto make a measurement of the concentration of two different gases (N1and N2 respectively). More generally, the invention relates to a devicecomprising 2×n emitters to make a measurement of the concentration of ndifferent gases.

FIG. 4 shows a second example embodiment of a device for measuring thegas concentration according to the invention.

According to this second example, mirrors are used for the transmissionof radiation in the cavity. The radiation li (i=1, 2, 3, 4) that entersthe cavity is reflected by mirrors ai, bi and ci in sequence. Mirrors biand ci are based on each side of the cavity C. The mirror ci is orientedso as to enable radiation ti to exit from the cavity through an orificeprovided for this purpose. For example, mirrors can be made using foldedand then polished metallic parts. This technology has the advantage thatit can easily be used and it avoids the need for a ZnSe lens. Protectiondeflectors DFi can be used to overcome possible dirt accumulation ofmeasurement systems by impurities carried by the gas flow. If theybecome dirty, the deflectors DFi can be cleaned using a high temperatureheating device which burns off the impurities.

According to the example embodiment shown in FIG. 4, the emitters Ei(i=1, 2, 3, 4) and the reception means Ri are placed on the same side ofthe cavity. Advantageously, the processing electronics for emission andfor reception of the radiation can then be placed at the same location.It is then easier to protect the optical emission and reception parts.Furthermore, the optical radiation path through the cavity is aforward-return type path. This path is then approximately twice as longas the path followed by the radiation in the previous cases (see FIGS.1, 2 and 3). This can significantly improve the sensitivity of themeasurement system.

FIG. 5 shows a third example embodiment of the device for measuring thegas concentration according to the invention.

According to this third embodiment, light conductors are used to guidethe various radiation li to the cavity C. Similarly, light conductorsare used to guide the different radiation ti output from the cavity tothe detection means. For example, the light conductors may be opticalfibers or endoscopes.

Radiation li output from the emitter Ei (i=1, 2, 3, 4) is thus routed tothe cavity C through a light conductor FEi and radiation ti output fromthe cavity is transmitted to the detector Di through a light conductorFRi. A lens Lli is used to focus radiation li in the cavity. Theradiation that enters the cavity is reflected by a mirror ci. The mirrorci is oriented so as to enable the radiation ti to exit from the cavitythrough an orifice provided for this purpose.

One advantage of this embodiment is to enable the cavity C to be movedaway from the optoelectronic processing zone. The temperature of theopto-electronic processing zone may then be made different from thetemperature of the cavity (for example, the temperature may be lower).This advantage is particularly useful to analyze exhaust gases in anautomobile.

Within the context of the invention described above, the emitters may begrouped on the same substrate in the form of a module or a matrix ofemitters. The emitters are pumped by a network of laser diodes with awavelength equal to approximately 800 nm. The dimensions of the networkof laser diodes are approximately equal to the dimensions of the pumplaser emitters. A module or a matrix is made after epitaxy and after themirrors have been made, either by lithography and etching to clearemitting areas facing the active areas of the pump emitters, or by usinga metallic mask to mask areas that must not emit light (with etchedholes).

The embodiment of the invention shown in FIG. 5 comprises four emittersand is a means of making a measurement of the concentration of twodifferent gases. The invention also relates to the case in which thedevice comprises 2×n emitters and can be used to make a measurement ofthe concentration of n different gases. Advantageously, the number n canbe fairly high (for example equal to 10), due to the large choice ofwavelengths within the 3 μm–6 μm range.

According to the embodiments of the invention described in FIGS. 3, 4and 5, the measurement device includes one emitter for which theemission spectrum is outside the absorption band, for each gas for whichthe concentration is to be measured. However, the invention also relatesto the case in which the number of emitters for which the emissionspectrum is outside the gas absorption band is less than the number ofgases for which the concentration is to be measured. For example, asingle emitter with an emission spectrum outside the gas absorption bandmay be used to measure the concentrations of several different gases.

1. A device for measuring a gas concentration, comprising: a cavity (C)containing at least one gas for which the concentration is to bemeasured, at least one first emitter (E1) composed of an opticalmicro-cavity pumped by optical pumping means and for which the emissionspectrum is within the gas absorption band, at least one second emitter(E2) composed of an optical micro-cavity pumped by optical pumping meansand for which the emission spectrum is outside the gas absorption band,a reception means (Ri) for measuring the optical intensity (I) of afirst radiation output from the first emitter (E1) and transmittedthrough the cavity (C), and the optical intensity (I₀) of a secondradiation output from the second emitter (E2) and transmitted throughthe cavity (C), and a processing circuit (T) for measuring the gasconcentration (N) starting from the optical intensity (I) of the firstradiation and the optical intensity (I₀) of the second radiation.
 2. Thedevice according to claim 1, characterized in that it comprises a firstoptical element (Lli) located on a first wall of the cavity to enablethe radiation (li) output from an emitter (Ei) to penetrate into thecavity (C) and a second optical element (Lti) located on a second wallof the cavity located facing the first wall to enable the radiation thatpenetrated into the cavity to leave the cavity (C).
 3. The deviceaccording to claim 1, characterized in that the cavity (C) comprises afirst opening to enable the radiation (li) output from an emitter (Ei)to penetrate into the cavity, in that the cavity includes a system ofmirrors (ai, bi, ci) to propagate the radiation inside the cavity and inthat the cavity comprises a second opening, close to the first opening,to enable the radiation propagated by the system of mirrors to leave thecavity (C).
 4. The device according to claim 3, characterized in thatthe system of mirrors (ai, bi, ci) is configured such that the path ofthe radiation that penetrates inside the cavity is at least anapproximately forward-return path between the first and the secondopening.
 5. The device according to claim 3, characterized in that themirrors (ai, bi, ci) are folded and polished metallic parts.
 6. Thedevice according to claim 3, characterized in that it comprises a lightconductor (FEi) to guide the radiation output from an emitter (Ei) tothe first opening and a light conductor to guide the radiation outputfrom the second opening to a reception means (Ri).
 7. The deviceaccording to claim 6, characterized in that the light conductor is anoptic fiber or an endoscope.
 8. The device according to claim 5,characterized in that the emitters are grouped in the form of modules ormatrices.
 9. The device according to claim 1, characterized in that theoptical pumping means are composed of laser diodes.
 10. The deviceaccording to claim 1, characterized in that the reception means comprisea first reception means to measure the optical intensity of the firstradiation (I) and a second reception means to measure the opticalintensity of the second radiation (I₀).
 11. The device according toclaim 1, characterized in that the reception means comprise at least onereception means activated selectively to measure either the opticalintensity of the first radiation (I), or the optical intensity of thesecond radiation (I₀).
 12. The device according to claim 1,characterized in that the deflectors (DFi) are placed inside the cavity(C).
 13. The device according to claim 1, characterized in that theoptical micro-cavities of the first (E1) and/or second (E2) emittercomprise an active region manufactured from semiconducting materialssuch as CdHgTe, GaAlN, AlBN, GaAlAs, GaAsSb, GaAlSb, or with differentfamilies of semiconducting alloys in the II–VI family (compounds of Cd,Zn, Hg, Mn, Mg with Se, S, Te), or the III–V family (Ga, Al, In, B withN, As, P, Sb).
 14. A device for checking the operation of a catalyticelement in an exhaust line of an automobile vehicle, characterized inthat it comprises a device according to any one of the above claims.